Method of motion tracking

ABSTRACT

A motion of a model including a joint, at least one body part that rotates with respect to the joint, and a plurality of inertial sensors attached to each body part and measuring a rotational motion of the body part is tracked by obtaining a rotational matrix of a sensor coordinate system fixed to each of the inertial sensors with respect to an inertial coordinate system fixed to ground, by using a signal measured by the inertial sensor attached to each body part; obtaining a rotational matrix of the sensor coordinate systems with respect to a body part coordinate system fixed to each body part, by using an obtained rotational matrix value of the sensor coordinate system; obtaining a rotational matrix of each body part coordinate system with respect to the inertial coordinate system, by using a calculated rotational matrix of the sensor coordinate systems; and calculating a joint variable with respect to each body part, by using a calculated rotational matrix of the body part coordinate system.

CROSS-REFERENCE TO RELEATED APPLICATIONS

This application is a National Phase entry of PCT Application No.PCT/KR2012/001119, filed on Feb. 15, 2012, which claims priority under35 U.S. C. §119(e), 120 and 365(c) to Korean Patent Application No.10-2011-0049804, filed on May 25, 2011, the entire disclosure of whichis incorporated herein by refernce for all purposes.

TECHNICAL FIELD

The present invention relates to a motion tracking method, and moreparticularly, to a motion tracking method which can accurately track amotion even when an inertial measurement unit (IMU) is attached to ahuman body in any directions because there is no need to accuratelymatch the direction of a sensor coordinate system of each IMU with thedirection of a body part coordinate system.

BACKGROUND ART

Motion tracking or motion capture is a method of tracking a motion of ahuman body and is technology used for various fields such as film oranimation production, sports motion analysis, medical rehabilitation,etc.

As a motion tracking method, there is an optical method and a magneticfield method. In the optical method, a reflection marker is attached toa human body and an infrared ray is irradiated onto the reflectionmarker by using an infrared camera so as to receive a light reflectedfrom the reflection marker. Also, in the magnetic field method, amagnetic member is attached to the body of a user and when the usermoves in a field where a magnetic field is formed, a motion of themagnetic member is determined by a change in the magnetic field.Recently, since a sensor is made small and light due to the developmentof micro-electromechanical systems (MEMS) technology, a method of usingan inertial measurement unit (IMU) has been introduced. The IMU is anapparatus for measuring an inertial force acting on an object moved byan applied acceleration. By measuring translational inertia, rotationalinertia, terrestrial magnetism, etc. of an object to be measured,various motion information such as acceleration, speed, direction,distance, etc. of the object may be provided.

FIG. 1 illustrates a state in which a plurality of inertial sensors Sare attached to a human body which has joints and body parts. FIG. 2 isa view schematically illustrating a kinematical model of the human bodyillustrated in FIG. 1. FIG. 3 is a view for explaining coordinatesystems used for the kinematical model of FIG. 2. As illustrated in FIG.2, a human body may be represented as a kinematical model having jointsJ1, J2, and J3 and body parts 1, 2, 3, and 4 that are rotatable withrespect to the joints J1, J2, and J3. The inertial sensors S1, S2, S3,and S4 are not attached to the joints J1, J2, and J3, but to the bodyparts 1, 2, 3, and 4 such as the trunk, an upper arm, a lower arm, andthe pelvis, respectively.

Thus, information regarding a motion of a human body may be obtained byusing the inertial sensors S1, S2, S3, and S4 attached to the body parts1, 2, 3, and 4. A motion of a human body may be tracked by processingthe information.

However, when the inertial sensors S1, S2, S3, and S4 are attached tothe body parts 1, 2, 3, and 4, as illustrated in FIG. 3, the directionsof coordinate systems {A and B} fixed to each of the body parts 1, 2, 3,and 4 need to be accurately matched with the directions of coordinatesystems {S_(A) and S_(B)} of each of the inertial sensors S1, S2, S3,and S4. However, since the surface of a human body is uneven, it ispractically almost impossible to match the directions of the coordinatesystems. Thus, when a motion is to be tracked by using the inertialsensors S, there is a need to match the directions of the coordinatesystems.

DISCLOSURE Technical Problem

Since the surface of a human body is uneven, it is practically almostimpossible to match the directions of the coordinate systems. Thus, whena motion is to be tracked by using the inertial sensors S, there is aneed to match the directions of the coordinate systems.

Technical Solution

According to an aspect of the present invention, a method of tracking amotion of a model including a joint, at least one body part that rotateswith respect to the joint, and a plurality of inertial sensors attachedto each body part and measuring a rotational motion of the body partincludes a sensor posture measurement operation of obtaining arotational matrix of a sensor coordinate system fixed to each of theplurality of inertial sensors with respect to an inertial coordinatesystem fixed to ground, by using a signal measured by the inertialsensor attached to each body part; a rotational matrix conversionoperation of obtaining a rotational matrix of the sensor coordinatesystems with respect to a body part coordinate system fixed to each bodypart, by using a rotational matrix value of the sensor coordinate systemobtained in the sensor posture measurement operation; a body partposture calculation operation of obtaining a rotational matrix of eachbody part coordinate system with respect to the inertial coordinatesystem, by using a rotational matrix of the sensor coordinate systemscalculated in the rotational matrix conversion operation; and a jointvariable calculation operation of calculating a joint variable withrespect to each body part, by using a rotational matrix of the body partcoordinate system calculated in the body part posture calculationoperation.

Advantageous Effects

According to the present invention, since the rotational matrixconversion operation of obtaining a rotational matrix of the sensorcoordinate systems with respect to the body part coordinate systemsfixed to the body parts, respectively, by using the rotational matrix ofthe sensor coordinate systems obtained in the sensor posture measurementoperation is provided, there is no need to accurately match thedirection of the body part coordinate system with the direction of thesensor coordinate system of each of the inertial sensor. Thus, accuratemotion tracking may be possible even when the inertial sensors areattached to a human body in any directions.

DESCRIPTION OF DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is an image illustrating a state in which a plurality of inertialsensors are attached to a human body which has joints and body parts;

FIG. 2 is a view schematically illustrating a kinematical model of thehuman body illustrated in FIG. 1;

FIG. 3 is a view for explaining coordinate systems used for thekinematical model of FIG. 2;

FIG. 4 is a flowchart for explaining a motion tracking method accordingto an embodiment of the present invention;

FIG. 5 is a view for explaining necessary coordinate systems used forthe motion tracking method of FIG. 4 and a rotational matrix among thecoordinate systems; and

FIG. 6 is a view for explaining necessary rotational matrix for themotion tracking method of FIG. 4.

BEST MODE

According to an aspect of the present invention, a method of tracking amotion of a model including a joint, at least one body part that rotateswith respect to the joint, and a plurality of inertial sensors attachedto each body part and measuring a rotational motion of the body partincludes a sensor posture measurement operation of obtaining arotational matrix of a sensor coordinate system fixed to each of theplurality of inertial sensors with respect to an inertial coordinatesystem fixed to ground, by using a signal measured by the inertialsensor attached to each body part; a rotational matrix conversionoperation of obtaining a rotational matrix of the sensor coordinatesystems with respect to a body part coordinate system fixed to each bodypart, by using a rotational matrix value of the sensor coordinate systemobtained in the sensor posture measurement operation; a body partposture calculation operation of obtaining a rotational matrix of eachbody part coordinate system with respect to the inertial coordinatesystem, by using a rotational matrix of the sensor coordinate systemscalculated in the rotational matrix conversion operation; and a jointvariable calculation operation of calculating a joint variable withrespect to each body part, by using a rotational matrix of the body partcoordinate system calculated in the body part posture calculationoperation.

The model may be a human body including a trunk, a pelvis coupled to thetrunk through a joint, an upper arm coupled to the trunk through ajoint, and a lower arm coupled to the upper arm through a joint.

The inertial sensor may be a sensor capable of measuring translationalinertia, rotational inertia, and terrestrial magnetism.

The inertial sensor may transmit a measured signal to the outsidewirelessly.

The inertial sensor may be manufactured according tomicro-electromechanical systems (MEMS) technology to be small and light.

MODE FOR INVENTION

The attached drawings for illustrating exemplary embodiments of thepresent invention are referred to in order to gain a sufficientunderstanding of the present invention, the merits thereof, and theobjectives accomplished by the implementation of the present invention.Hereinafter, the present invention will be described in detail byexplaining exemplary embodiments of the invention with reference to theattached drawings. Like reference numerals in the drawings denote likeelements.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Expressions such as “atleast one of,” when preceding a list of elements, modify the entire listof elements and do not modify the individual elements of the list.

FIG. 4 is a flowchart for explaining a motion tracking method accordingto an embodiment of the present invention. FIG. 5 is a view forexplaining necessary coordinate systems used for the motion trackingmethod of FIG. 4 and a rotational matrix among the coordinate systems.

Referring to FIGS. 4 and 5, the motion tracking method according to thepresent embodiment is a method of tracking a motion of a human body byattaching a plurality of inertial sensors S to a dynamic model formed ofjoints and body parts, for example, a moving object such as a humanbody. The motion tracking method according to the present embodimentincludes a sensor posture measurement operation S100, a rotationalmatrix conversion operation S200, a body part posture calculationoperation S300, and a joint variable calculation operation S400.

Before explaining the motion tracking method, a human body model usedfor explaining a process of adopting the motion tracking methodaccording to the present embodiment will be first described.

The human body model, as illustrated in FIG. 2, includes the joints J1,J2, and J3, at least one of the body parts 1, 2, 3, and 4 that rotateswith respect to at least one of the joints J1, J2, and J3, and theinertial sensors S1, S2, S3, and S4 attached to the body parts 1, 2, 3,and 4 and measuring rotational motions of the body parts 1, 2, 3, and 4,respectively. In the present embodiment, the inertial sensors S1, S2,S3, and S4 are sensors manufactured according to MEMS technology and mayeach be inertial measurement units (IMU) capable of measuringtranslational inertia, rotational inertia, and terrestrial magnetism. Ameasured signal may be transmitted to the outside wirelessly.

In detail, the human body model, as illustrated in FIG. 2, includes thetrunk 1, the pelvis 4 coupled to the trunk 1 through the waist joint J3,the upper arm 2 coupled to the trunk 1 through the shoulder joint J1,and the lower arm 3 coupled to the upper arm 2 through an elbow jointJ2. Accordingly, the trunk 1, the upper arm 2, the lower arm 3, and thepelvis 4 form the body parts 1, 2, 3, and 4 of the human body model,respectively. The waist joint J3 between the trunk 1 and the pelvis 4has a joint variable Q3 of three degrees of freedom. The shoulder jointJ1 between the upper arm 2 and the trunk 1 has a joint variable Q1 ofthree degrees of freedom. The elbow joint J2 between the lower arm 3 andthe upper arm 2 has a joint variable Q2 of three degrees of freedom. Thejoint variables Q1, Q2, and Q3 denote a rotational motion of each of thejoints J1, J2, and J3 and are well known to one skilled in the art sothat detailed descriptions thereof are omitted herein.

In the present embodiment, a variety of coordinate systems are used. Asillustrated in FIG. 5, there are an global coordinate system {0} that isfixed to the ground as a inertial coordinate system, sensor coordinatesystems {S_(B), S_(A), S_(E), and S_(W)} that are fixed to the inertialsensors S1, S2, S3, and S4, respectively, and body part coordinatesystems {B, A, E, and W} that are fixed to the body parts 1, 2, 3, and4, respectively.

An example of applying the motion tracking method to the above-describedhuman body model will now be described.

First, as the body parts 1, 2, 3, and 4 perform rotational motions,signals measured by the inertial sensors S1, S2, S3, and S4 attached tothe body parts 1, 2, 3, and 4, respectively, are wirelessly transmittedto the outside. A rotational matrix R⁰ _(SB), R⁰ _(SA), R⁰ _(SE), and R⁰_(SW) of the sensor coordinate systems {S_(B), S_(A), S_(E), and S_(W)}fixed to the inertial sensors S1, S2, S3, and S4, respectively, isobtained with respect to the global coordinate system {0} by using themeasured signals. (Sensor posture measurement operation S100)

Next, a rotational matrix R_(B) ^(SB), R_(A) ^(SA), R_(E) ^(SE), andR_(W) ^(SW) of the sensor coordinate systems {S_(B), S_(A), S_(E), andS_(W)} is obtained with respect to the body part coordinate systems {B,A, E, and W} fixed to the body parts 1, 2, 3, and 4, respectively, byusing the rotational matrix R⁰ _(SB), R⁰ _(SA), R⁰ _(SE), and R⁰ _(SW)of the sensor coordinate systems {S_(B), S_(A), S_(E), and S_(W)}obtained in the sensor posture measurement operation S100.

Equation (1), derived from an equation of a motion between the trunk 1and the upper arm 2, is used. Here, R_(A) ^(B) denotes a rotationalmatrix between the body part coordinate system {B} fixed to the trunk 1and the body part coordinate system {A} fixed to the upper arm 2.R ⁰ _(SA) R _(A) ^(SA) =R ⁰ _(SB) R _(B) ^(SB) R _(A) ^(B)  (1)

Since the unknowns in Equation (1) are R_(A) ^(SA) and R_(B) ^(SB), theother variables R⁰ _(SA), R⁰ _(SB), and R_(A) ^(B) are constants inorder to obtain the unknowns R_(A) ^(SA), and R_(B) ^(SB). In thepresent embodiment, the other variables R⁰ _(SA), R⁰ _(SB), and R_(A)^(B) are determined after a human body model performs threepredetermined motions. The three motions may be, for example, a motionof stretching an arm forward, a motion of raising an arm upwards, and amotion of stretching an arm laterally. When Equation (1) is applied tothe three motions, the following Equation (2) is produced. Here, R⁰_(SA)(k), R⁰ _(SB)(k), and R_(A) ^(B)(k) are constant values determinedto the three motions.R ⁰ _(SA)(k)R _(A) ^(SA) =R ⁰ _(SB)(k)R _(B) ^(SB) R _(A) ^(B)(k), wherek=1,2, and 3  (1)

Likewise, by applying an equation similar to Equation (2) to the trunk 1and the pelvis 4 and an equation similar to Equation (2) to the trunk 1and the lower arm 3, the rotational matrix R_(B) ^(SB), R_(A) ^(SA),R_(E) ^(SE), and R_(W) ^(SW) of the sensor coordinate systems {S_(B),S_(A), S_(E), and S_(W)} is obtained with respect to the body partcoordinate systems {B, A, E, and W}. (Rotational matrix conversionoperation S200)

Next, by using the rotational matrix R_(B) ^(SB), R_(A) ^(SA), R_(E)^(SE), and R_(W) ^(SW) of the sensor coordinate systems {S_(B), S_(A),S_(E), and S_(W)} calculated in the rotational matrix conversionoperation S200, a rotational matrix R_(B) ⁰, R_(A) ⁰, R_(E) ⁰, and R_(W)⁰ of each of the body part coordinate systems {B, A, E, and W} isobtained with respect to the global coordinate system {0}. In thefollowing description, the rotational matrix R_(B) ⁰, R_(A) ⁰, R_(E) ⁰,and R_(W) ⁰ of the body part coordinate systems {B, A, E, and W} isexpressed by a rotational matrix {T_(t), T_(u), T_(f), T_(p)} asillustrated in FIG. 6. Here, the following Equations (3) through (6) areused therefor.T _(t) =R _(B) ⁰ =R ⁰ _(SB) R _(B) ^(SB)  (3)T _(u) =R _(A) ⁰ =R ⁰ _(SA) R _(A) ^(SA) =R ⁰ _(SB) R _(B) ^(SB) R _(A)^(B)  (4)T _(f) =R _(E) ⁰ =R ⁰ _(SE) R _(E) ^(SE) =R ⁰ _(SB) R _(B) ^(SB) R _(A)^(B) R ^(A) _(E)  (5)T _(p) =R _(W) ⁰ =R ⁰ _(SW) R _(W) ^(SW) =R ⁰ _(SB) R _(B) ^(SB) R _(W)^(B)  (6)

Here, R^(A) _(E) denotes a rotational matrix between the body partcoordinate system {A} fixed to the upper arm 2 and the body partcoordinate system {E} fixed to the lower arm 3. R_(W) ^(B) denotes arotational matrix between the body part coordinate system {B} fixed tothe trunk 1 and the body part coordinate system {W} fixed to the pelvis4. (Body part posture calculation operation S300)

Finally, by using the rotational matrix {T_(t), T_(u), T_(f), T_(p)} ofthe body part coordinate systems {B, A, E, and W} calculated in the bodypart posture calculation operation S300, the joint variables Q1, Q2, andQ3 are calculated with respect to the joints J1, J2, and J3,respectively. The following Equation (7) is derived from the kinematicsfrom the trunk 1 to the upper arm 2, the following Equation (8) isderived from the kinematics from the upper arm 2 to the lower arm 3, andthe following Equation (9) is derived from the kinematics from the trunk1 to the pelvis 4.T _(t) =T _(u) T _(su) e ^(Q1) T _(ts)  (7)T _(u) =T _(f) T _(ef) e ^(Q2) T _(ue)  (8)T _(t) =T _(p) T _(wp) e ^(Q3) T _(tw)  (9)

Here, T_(ts) denotes a matrix in consideration of a distance from thetrunk 1 to the shoulder joint J1. T_(su) denotes a matrix inconsideration of a distance from the shoulder joint J1 to the upper arm2. T_(ue) denotes a matrix in consideration of a distance from the upperarm 2 to the elbow joint J2. T_(ef) denotes a matrix in consideration ofa distance from the shoulder joint J1 to the lower arm 3. T_(tw) denotesa matrix in consideration of a distance from the trunk 1 to the waistjoint J3. T_(wp) denotes a matrix in consideration of a distance fromthe waist joint J3 to the pelvis 4.

To summarize Equations (7) through (9), the following Equations (10)through (12) are derived,Q1=log((T _(u) T _(su))⁻¹ T _(t) T _(ts) ⁻¹)  (10)Q2=log((T _(f) T _(ef))⁻¹ T _(u) T _(ue) ⁻¹)  (11)Q3=log((T _(p) T _(wp))⁻¹ T _(t) T _(tw) ⁻¹)  (12)

The joint variables Q1, Q2, and Q3 of the joints J1, J2, and J3 may beobtained from the Equations (10) through (12). (Joint variablecalculation operation S400)

In the above-described motion tracking method, since the rotationalmatrix conversion operation S200 for obtaining the rotational matrixR_(B) ^(SB), R_(A) ^(SA), R_(E) ^(SE), and R_(W) ^(SW) of the sensorcoordinate systems {S_(B), S_(A), S_(E), and S_(W)} with respect to thebody part coordinate systems {B, A, E, and W} respectively fixed to thebody parts 1, 2, 3, and 4, by using the rotational matrix R⁰ _(SB), R⁰_(SA), R⁰ _(SE), and R⁰ _(SW) of the sensor coordinate systems {S_(B),S_(A), S_(E), and S_(W)} obtained in the sensor posture measurementoperation S100 is provided, there is no need to accurately match thedirections of the body part coordinate systems {B, A, E, and W} fixed tothe body parts 1, 2, 3, and 4, respectively, with the directions of thesensor coordinate systems {S_(B), S_(A), S_(E), and S_(W)} of theinertial sensors S1, S2, S3, and S4. Thus, accurate motion tracking maybe possible even when the inertial sensors S1, S2, S3, and S4 areattached to a human body in any directions.

Also, in the motion tracking method, since the joint variablecalculation operation S400 for calculating the joint variables Q1, Q2,and Q3 with respect to each of the joints J1, J2, and J3, by using therotational matrix {T_(t), T_(u), T_(f), T_(p)} of the body partcoordinate systems {B, A, E, and W} calculated in the body part posturecalculation operation S300 is provided, the joint variables Q1, Q2, andQ3 may be obtained by using the rotational matrix {T_(t), T_(u), T_(f),T_(p)} of the joints J1, J2, and J3.

Also, in the motion tracking method, since the inertial sensors S1, S2,S3, and S4 for measuring translational inertia, rotational inertia, andterrestrial magnetism are used, compared to a conventional opticalmethod or a magnetic field method, response speeds of the inertialsensors S1, S2, S3, and S4 for measuring rotational motions of the bodyparts 1, 2, 3, and 4 are fast. Accordingly, even when a human body modelmoves fast, accurate motion tracking is possible.

In addition, in the motion tracking method, since the inertial sensorsS1, S2, S3, and S4 capable of transmitting measured signals to theoutside wirelessly are used, it is easy to attach the inertial sensorsS1, S2, S3, and S4 to a human body and complicated wiring is not neededso that a motion of a human body, which is an object of motion tracking,may be tracked without hindrance.

In the motion tracking method, since the inertial sensors S1, S2, S3,and S4 are manufactured according to MEMS technology to be small andlight, the inertial sensors S1, S2, S3, and S4 are easy to carry andprovide comfort like they are not attached to a human body.

In the present embodiment, although motion tracking is performed withrespect to a human body model, mainly to the upper body including thetrunk 1, the pelvis 4 coupled to the trunk 1, the upper arm 2 coupled tothe trunk 1, and the lower arm 3 coupled to the upper arm 2, the motiontracking may also be applied to a human body model including the lowerbody such as thigh, calf, foot, etc.

As described above, according to the present invention, since therotational matrix conversion operation of obtaining a rotational matrixof the sensor coordinate systems with respect to the body partcoordinate systems fixed to the body parts, respectively, by using therotational matrix of the sensor coordinate systems obtained in thesensor posture measurement operation is provided, there is no need toaccurately match the direction of the body part coordinate system withthe direction of the sensor coordinate system of each of the inertialsensor. Thus, accurate motion tracking may be possible even when theinertial sensors are attached to a human body in any directions.

While this invention has been particularly shown and described withreference to exemplary embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

The invention claimed is:
 1. A method of tracking a motion of a modelcomprising a joint, body parts that rotate with respect to the joint,and inertial sensors attached to each of the body parts to measure arotational motion of each of the body parts, the method comprising:attaching the inertial sensors to each of the body parts in differentorientations with respect to one or more of each of the body parts andground, wherein the inertial sensors are attached to each of the bodyparts without matching a direction of the body part coordinate systemwith a corresponding direction of the sensor coordinate system of eachof the inertial sensors; tracking motion of the body parts using theinertial sensors; capturing a motion of the body parts using a jointvariable and a rotational matrix of the body part coordinate system;outputting the calculated joint variable as a captured motion of thebody parts; a sensor posture measurement operation of obtaining therotational matrix of the sensor coordinate system fixed to each of theinertial sensors with respect to an inertial coordinate system fixed tothe ground using a signal measured by the inertial sensors; a rotationalmatrix conversion operation of obtaining the rotational matrix of thesensor coordinate systems with respect to the body part coordinatesystem fixed to each of the body parts, using a rotational matrix valueof the sensor coordinate system obtained in the sensor posturemeasurement operation; a body part posture calculation operation ofobtaining a rotational matrix of each body part coordinate system withrespect to the inertial coordinate system using a rotational matrix ofthe sensor coordinate systems calculated in the rotational matrixconversion operation; using a rotational matrix value of the sensorcoordinate system obtained in the obtaining of the rotation matrix ofthe sensor coordinate system fixed to each of the inertial sensors withrespect to the inertial coordinate system fixed to the ground, and asignal measured by the inertial sensor attached to each of the bodyparts, as each of the body parts performs three or more motions; and ajoint variable calculation operation of calculating a joint variablewith respect to each body part, using a rotational matrix of the bodypart coordinate system calculated in the body part posture calculationoperation, wherein the obtaining of the rotational matrix of the sensorcoordinate systems with respect to the body part coordinate system fixedto each of the body parts comprises using a signal measured by theinertial sensor attached to each of the body parts as each of the bodyparts performs three or more motions, wherein the calculating of thejoint variable further comprises transforming motion data, measured bythe inertial sensor with respect to the sensor coordinate system fixedto each of the inertial sensors, using the rotational matrix of the bodypart coordinate system to produce the calculated joint variable, andwherein the model is a human body comprising a trunk, a pelvis coupledto the trunk through a joint, an upper arm coupled to the trunk througha joint, and a lower arm coupled to the upper arm through a joint, awaist joint between the trunk and the pelvis comprising a joint variableof three degrees of freedom, a shoulder joint between the upper arm andthe trunk comprising a joint variable of three degrees of freedom, andan elbow joint between the lower arm and the upper arm comprising ajoint variable of three degrees of freedom.
 2. The method of claim 1,wherein the inertial sensor is capable of measuring translationalinertia, rotational inertia, and terrestrial magnetism.
 3. The method ofclaim 1, wherein the inertial sensor is configured to wirelesslytransmit a measured signal.
 4. The method of claim 1, wherein theinertial sensor is manufactured according to micro-electromechanicalsystems (MEMS) technology.